Control system for electric furnaces



, 0. A. COLBY. CONTROL SYSTEM FOR ELECTRIC FURNACES. APPLICATION FILED FEB-15,1919.

1,349,363. Patented Aug. 10, 1920.

far/76 06 Furnace j I m I gl WITNESSES: INVENTOR ea W 6 ORA A. COLBY, OF LABIM & MANUFACTURING COMPANY,

ER, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC A CORPORATION OF PENNSYLVANIA.

CONTROL SYSTEM FOR ELECTRIC FURNACES.

Specification of Letters Patent.

Patented Aug. 10, 1920.

Application filed February 15, 1919. Serial No. 277,308.

To all whom it may concern:

Be it known that I. ORA A. COLBY, a citizen of the. United States,

and a resident of Larimer, in the county of Vestmoreland and State of Pennsylvania, have invented a new and useful Improvement in Control Systems for Electric Furnaces, of which the following is a specification.

My invention relates to and particularly electric-resistance One object of my electric furnaces,

to methods of control for furnaces.

invention is to provide an automatic control system for electric-re sistance furnaces 'where t he resistor has anegative temperature-resistance coefficient.

Another object of my invention is to provide an automatic control system for electric-resistance furnaces that will be simple, relatively inexpensive, durable and easily kept in working order.

Another object of my invention is to provide acontrol system that will enable the operator to maintain any ture 1n the furnace within desired temperaa relatively small range of temperature, and that will enable him to easily change the working temperature in the furnace to any other desired value.

In operating electric-resistance furnaces that have a negative-temperature-1esistance coeflicient, it is necessary to apply a voltage to the cold resistor that is much higher than is required when the resistor has been heated to its working temperature. here a transormer is used to reduce value required by voltage may be obtained the voltage to the the furnace. this variable by changing the number of effective turns in the primary winding of the transformer. and my invention relates to the method this object.

In practising my inven plurality of electromagnetically of accomplishing tion, I provide a operated switches to control the connection of the electric supply circuit to the primary winding, actuating these switches either by circuit-closing relays that are controlled by thermostats or by selves.

the thermostats them- The thermostats are caused to operate by a plurality of heating coils of different heating capacities that to the secondary winding rent transformer which is current flowing through t are connected of a suitable curenergized by the he resistor of the wlse, so as to preclude more than one switch being closed at one time and thereby short clrcuiting a section of the transformer.

Figure 1 of the drawing is a diagram of apparatus and circuits embodying circuitclosing relays to actuate the electromagnetic switches, the relays being controlled by thermostats. Fig. 2 is a diagram of apparatus and clrcuits embodying double-contact thermostats for controlling the electromagnetic switches.

Referring to Fig.1, an electric supply circuit 10 energizes a transformer A through a switch 11, the current flowing through the conductor 12 to the primary winding of the transformer A at either the point 20 or the point 21 (according to the position of the switch member 19). then through the active portion of the winding and out through one of the conductors 13. 11 and 15. The conductors 13, 11 and 15 are connected to the other conductor 12 of the supply circuit 10 through electromagnetic switches 16, 17 and 18. respectively. The single-pole. doublethrow switch member 19 may be manually thrown to engage nal 20 or the contact terminal :21. according to thenumber of turns of the primary winding to be included in the circuit. The secondary winding of the transformer A is divided into two halves that may be connected in series or in parallel by a switch 22 and delivers current to the furnace B through the conductors 25 and Q4 and the protective circuit-breaker 23.

A plurality of circuit-closing relays 26, 24'. and 28, close and open the circuits through the coils of the respective electromagnetic switches 16, 17. and 18. One of the terminals of each of the coils of switches 16. 17 and 18. is connected to the conductor 12 by means of the conductor 29, while the other terminals of the coils are connected to corresponding stationary contact members of the relays 26. 27 and 28, respectively. I The other corresponding contact members of the relays 26, 27 and 28. are

either the contact termi- The . it desired.

connected to a conductor 33. that is constrip lO is connected to the fixed end (it a like strip 42* ot a thermostat t2 by means of a conductor 41. A contact terminal 4:3 is connected to'the conductor 12' of the supply circuit by means of a conductor ll. One terminal of the coil of relay 28 is connected to the conductor 33, while the other termiml of the coil is connected to a contact terminal 45 by means of a conductor 4%. The fixed end ot the strip l'a' of a thermostat at? is connected, by means of a conductor l8,

to the fixed end of the strip -l9 of a ther mostat 9. The contact terminal of a thermostat T9 is connected, by means of conductor let, to cohductor 12 of the supply circuit. The bi-metallic strips 36, -l2 and 49 engage the respective contact terminals 35, 4:3 and 50 when there is no current'fiowing through the heating coils of the thermostats, while the strips leO and all oil the thermostats t0 and atl'are out of engagement with the contact terminals 39 and T5, under like conditions.

Heating coils 36", t0", 42 t? and t9 are mounted in the casings of the respective thermostats 36, l0, l2, l-T and 49 and are connected, in series, across the secondary winding 51 of a current transformer 51 that is connected to the-conductor 2t leading to one contact terminal of the furnace B.

The heating coils may be connected to the conductor 24: in any other suitable manner, A rheostat 52 is connected in parallel with the heating coils and serves as a means for regulating the amount of current fiowing through them The heating (OllS 36. T0", ta at? and t9 are so designed that the coil 36 has, relatively, the

largest amount of energy loss, t0 the next largest, and so on down to 4L9, which has relatively the maallest energy loss of the group of heating coils.

Fig. 2 shows two double-contact thermostats and one single-contact thermostat to control the operation of the electromagnetic switches 16, 1'4' and 18. One terminal ot the coil of switch 16 is connected to the supply conductor 12* by means of conductor 29, and the other terminal is connected to-the contact term nal of a thermostat 60 by means tot a conductor (it. One terminal of each of the coilsot the switches 1'4 and 18 raaaaea is connected to the same conductor 29, and

the other terminal of the coil of switch 17 I The fixed end of a strip 62 is connected to the conductor 12 otthe supply circuit 10.

A. contact terminal 72 is connected to the fixed end of a strip 63* by means of a conductor it. The free ends of the bi-metallic strips 60*, 62* and 63 engage, respectively, the contact terminals 65, T0 and 7 3 when thethermostats are cold. .The heating coils 60 62 and 63. are connected in series and across the terminals of the winding 51 of the current transformer 51*. The heating coils are so designed that coil 60? has relatively the largestwattloss, the C01lS 62 and 63 having each a relatively, and progressively, smaller energy loss.

If the positionfof the various parts is as shown in Fig. 1 and it is desired to place the system in operation, the procedure is as follows: Throw the switch 22 to the left to connect the two halves of the secondary winding of the transformer A in parallel and throw the switch member 19 to engage the contact terminal 20. Then close the switch 11 in the supply circuit 10. Current will fiow through acontrol circuit, comprisin the following: from conductor 12 through conductor 37, thermostat strip 36, contact terminal 35, conductor 3t, the coil of relay 26 and conductor 33, to conductor 12. This will cause relay 26 to close a circuit as follows: from conductor 29 through the coil of switch 16, conductor 30, contact terminals and contact bridging member of relay 26 to conductor 33, and thence to the other conductor 12 of the supply circuit 10. This will operate to close a circuit as follows: from the conductor 12, through the contact terminals and contact bridging member of the switch 16, conductor 13, a portion of the primary winding of the transformer A,'conductor :20 and switch member 19 to conductor 1:2. As the number of turns of the primary winding included in the circuit is relatively small, a relatively high secondary voltage will be applied to the terminals of the resistors of furnace B, and a current will flow through the furnace. resulting in a gradual increase of temperature of the resistor. As the temperature rises, the resistance will decrease, resulting in an increase of current and, therefore, a further rise of temperature. A current is induced in the secondary-winding 51 of the current transformer 51 that is proportional to the current flowing through the resistor of the Ittltl Illtl lllh ran

furnace B. This current flows through a circuit comprising a conductor 53, the heating coils 36", 40, 42 47 and 49*, connected in series, and a conductor 54, and, if the rheostat 52 is in circuit, a proportional part of this current will flow through it. As the heating coil 36 has relatively the maximum watts loss, the strip 36 will be caused to operate first. At a predetermined value of current in furnace B, the derived current in the transformer 51 and its circuit, as described above, will be such as to cause the strip 36 to be disengaged from its contact terminal 35 and thereby open the circuit through the coil of relay 26. This, in turn, will open the control circuit through the switch 16 and thereby disconnect the conductor 12 from the conductor 13 and from the primary winding of the transformer A.

The adjustment of the thermostat 40 and the watts loss in the heating coil 40 are such that the strip 40 will engage the contact terminal 39 as soon as the strip 36 is'disengaged from its contact terminal 35. A new control circuit will then be established comprising the conductor 44, contact terminal 43, strip 42 conductor 41, strip 40 contact terminal 39, conductor 38, the coil of relay 27, and conductor 33 to conductor 12. This will operate to close a circuit through relay 27 as follows: from conductor 12 through conductor 29, the coil of switch 17 conductor 31, contact terminals and contact bridging member of relay 27 and conductor 33 to conductor 12. This will cause the switch 17 to close and establish the following circuit; from conductor 12, through the contact terminals and contact bridging member of switch 17, conductor 14, the portion of the primary winding of transformer A, between conductor 14 and terminal 20, and switch member 19, to conductor 12. This circuit, therefore, comprises a-larger number of turns of wire ,of the primary winding, the secondary voltage of the transformer will be reduced, with a corresponding reduction in the current flowing through the resistor of v furnace B.

If the resistance of the furnace continues to decrease, the current flowing through the furnace! and through the conductor 24 will continue to increase and, therefore, the current in the winding 51 will increase until the heat generated in the heating coil 42 will cause the strip 42" to be disengaged from the contact terminal 43. This will open the circuit through the coil of relay27 and this, in turn, will cause switch 17 to open and deenergize the transformer 1 The adjustment of thermostat 47 is such that the strip 47 will engage the contact terminal 45, immediately after strip 42 is disengaged from contact terminal 43, and close a circuit through the coil of relay 28,'similar to that described for the other relays. This and hence,

will energize switch 18 and connect the conductor 12 to conductor 15 and, consequently, a still larger number of turns of the primary winding will be included in the circuit. The secondary voltage will, therefore, be again reduceed and, within normal limits, should be low enough to permit only such value of current to flow in the furnace B, as will maintain the desired temperature therein. If, however, the resistance of furnace B decreases still further, with a corresponding increase of current, the strip 49 will be disengaged from its contact terminal 50 and will open the circuit through the coil of relay 28. This will cause the switch 18 to open and disconnect the conductor 12 from the transformer, and, therefore, no current will flow through the furnace. In a relatively short time, the strip 49 will to again engage the contact terminal 50, thus restoring the circuit through the coil of relay 20*, resulting in switch 18 closing, and reenergizing the transformer, with the largest number of turns of the primary winding included in the circuit.

become cool enough i Should it be desired to operate the furnace at a higher temperature, the rheostat 52 may be connected in circuit and the. less the resistance connected across the conductors 53 and 54. the greater will be the current diverted from the heating coils of the thermostats. The current through the furnace B must, therefore, be greater to cause the thermostats to operate as described above and, hence, the temperature of the furnace will be higher. provided as a final safety device, to protect the resistor of furnace B in case of accident, and is not a part of my invention.

lVhen it is desired to omit the circuitclosing relays, the system shownin Fig. 2 may be used and the thermostats will operate to close and open the circuits through the coils of the electromagnetic switches themselves. Assume that switch 22 has been closed, that switch 19 engages the contact terminal 20 and that switch 11 is then closed. A control circuit will be established as follows: from conductor 12, through conductor 71, the strip 62, contact terminal 70, conductor 69, strip 60, contact terminal 65, conductor 64, the coil of switch 16, and conductor 29, to conductor 12f. his will cause the switch 16 to close and energize the transformer A, throughthe following circuit: from conductor 12*, through the contact terminals and contact bridging member of switch 16, conductor 13, a portion of the primary winding of transformer A, terminal 20, and switch member 19, to conductor 12. This will .cause a relatively high secondary voltage to be impressed on the resistor of furnace B and cause it to heat up. As the resistance decreases and the current increases, the derived current in the ourrent transformer 51 will also increase and the thermostat heating coils will heat up. As coil 60" has the relatively largest watt loss, it will cause the strip 60 to be disengaged from contact terminal 65 and to immediately engage contact terminal 68. This action opens the control circuit of switch 16, causing the conductor 12 to he disconnected from the primary winding of the transformer and closes the control circuit of switch 17, causing it to close and connect conductor 12- to conductor 14, and, therefore, to the primary winding of the transformer A, with a relatively larger num her of turns included in the circuit. This will reduce the voltage impressed on the resistor of the furnace and, hence,'tend to reduce the current or to maintain it at a constant value. llf the resistance of the furnace l3 decreases further, with a corresponding increase of current the heating coil 62 will cause the. strip te to be disengaged from contact terminal 70 and to engage cont-act terminal 72. This action will open the circuit through the coil of switch 17 and close the circuit through the coil of switch 18, comprising the following:

. from conductor 12, conductor 71, strip 6%,

contact terminal 72, conductor 74,.strip 63*, contact terminal 73, conductor 67, the "coil of switch18 and conductor 29, to conductor 12*. This will cause the switch 18 to close and connect the conductor 12 to the primary winding of the transformer A through conductor 15 and, therefore, a still larger number of turns is included in the circuit. This will reduce the voltage impressed on the furnace B and, therefore, will tend to. reduce the current flowing through the resistor or to keep it at a constant value, even if the resistance of the furnace decreases. Tf the resistance should decrease still more,

the derived current flowing through the winding 51 and the heating coils of the thermostats will cause the strip 63 to he disengaged from contact terminal 7 3. This will open the circuit through the coil of switch 18, causing this switch to open and interrupt the circuit through conductor 15 and, therefore, denergizing the transformer A. As no current'is flowing in the furnace and. thermostat heating coils, the lei-metallic strip 63 will immediately begin to cool and will soon reengage the contact terminal 73, restoring the circuit through the coil of switch 18, causing that switch to close and reenergize the transformer. The amount of current in the furnace 1B, and,

hence, the operating temperature may be adjusted by means ofrheostat 52, as described in connection with Fig. 1.

If a higher secondary voltage is desired at any time, switch member 19 may be so 'thrownas to engage the contact terminal 21 and thus cut out a part of thepr'imary naaaeea winding of the transformer Tf a still higher secondary voltage is required, switch 22 may be so thrown as to connect the two halves of the secondary winding of transformer A in series. These two switches are provided mainly to allow of securing a relatively large range of voltage adjustment to meet all conditions that may arise in the operation of such furnaces and are not parts of my invention.

While T have shown specific applications of my system, ll desire that only such limitations shall .be placed thereon as are imposed by the prior art or are specifically set current in proportion to that supplied to the main heating element, means for adjusting the ratio of the currentsin the two heating elements, and means actuated in response to variations of thevtemperature of the auxiliary heating element for regulating the temperature of the main heating element.

2. An electrical heating system comprising a main heatingelement having a nega tivetemperature-resistance coefficient, the temperature of which is to be regulated, an auxiliary heating element out of thermal communication with the main heating element and receiving current in proportion to that supplied to the main heating element, and means actuated in response to variations of the temperature of the auxiliary heating element for regulating the temperature of the main heating element.

3. An electrical heating system comprising a main heating element having a negativetemperature-resistance coeficient, the temperature of which is to be regulated, an electrical circuit supplying the same, an auxiliary heating element, means for supplying to the auxiliaryheating element a current derived from the circuit supplying the main heating element that is in direct proportion 6. An electrical heating system comprisheating element out of thermal communication with the main heating element for actu ating said control means.

5. An electrical heating system comprising a main heating element having a negatlve-temperature-resistance coefficient the ing a main heating element having a nega-- tive-temperature-resistance coefficient, the temperature of which is to be regulated, a transformer, an electrical circuit for energizing said transformer, circuit-closing devices for energizing predetermined portions of the primary winding of said transformer, temperature-actuated means for controlling said circuit-closing devices, an auxiliary heating element out of thermalcommunication with the main heating element for controlling said temperature-actuated control means, and means for supplying to the auxiliary heating element a current derived from the circuit supplying the main heating element that is in direct proportion to thecurrent in said circuit.

7. An, electrical heating systemcomprising a main heating element having a negative-temperature-resistance coefiiclent, the temperature of which is to be regulated, a transformer, an electrical circuit for supplying energy to said transformer, circuit-closng devices for energizing predetermined portions of the primary winding of said transformer, means for controlling said circuit-closing devices, said means being responsive to variations of current in the main heating element. f

8. An .electrical heating system comprising a main heating element having a negative-temperature-resistance coeflicient, the temperature-foilwhichis to be regulated, a transformer 'v-ing' a plurality of taps on the primarfi 'wi'nding, an electrical circuit for energiz ng said transformer, a pluralit of circuit closiiigg deviceq for rendering e fective, predetermined portions of the primary -winding "of. said transformer, a plurality of temperature-actuated means for controlling said"circuit-closing devices, said temperature-actuated control means being arranged to preclude more than one circuitclosing device being closed at one time.

iliary heating element a current derived 9. An electrical heating system comprising an electrical supply circuit, a transformer having a plurality of taps on the primary winding, a' main heating element having a negative-temperature-resistance coeflicient, the temperature of which is to be regulated, a plurality of circuit-closing devices for energizing portions of the winding of said transformer, means for causing said circuit-closin devices to operate in predetermined or er, an auxiliary heating element out of thermal communication with'the 1 main heating element for actuating said means, and means for supplying to the auxfrom the circuit supplying the main heating element. t

10. In an electrical heating system the combination with a transformer having a plurality of sections in the primary winding, an electrical circuit supplying said transformer, a main heating element having a negative temperature resistance coeflicient,

the temperature of which is to be regulated and receiving its current from the secondary of said transformer, circuit-closing devices for energizing predetermined portions of the primary winding of said transformer, tem

perature-actuated means for controlling said circuit-closing devices, and an auxiliary heating element, out of thermal communication with the main heating element, for controlling said temperature-actuated means, of means forsupplying to the auxiliary heating element a current derived from the circuit supplying the main heating element that is in proportion to the current in said heating element, and means for adjusting the proportion of the currents in the two heatin elements.

11. in electrical heating system comprising a transformer having a plurality of sections in the primary winding, ai -electrical circuit supplying energy to said transformer, a main heating element, having a negative temperature resistance coeflicient, the temperature of which is to be regulated, a plurality ofcircuit-closing devices for energizing portions of said transformers, a plurality of temperature-actuated means for 115 controlling said circuit-closing devices, an auxiliary heating element, out of thermal ---com mun'ication with the main heating element, having a plurality of sections of diftive-temperature'resistance coeficient, the temperature of which is to be regulated, an electrical circuit supplying said main heating element, an auxiliary heating element having a plurality of sections of difi'erent heating capacities, out of thermal communication with the main heating element and receiving current inrproportion to that supplied to the main heating element, and means actuated in response to variations of the auxiliary heating element for regulating the temperature of the main heating element.

13. An electrical heating system comprising a main heating element, having a negative-temperature-resistance coeflicient, the temperature of which is to be regulated, an electrical circuit supplying the same, an auxiliary heating element having a plurality of sections of difi'erent heating capacities, out of thermal communication with the main heating element and receiving ,current in I proportion to that supplied to the main heating element, means for adjusting the ratio of the currents in the two heating elements,

and means actuated in response to varia-,

X tions of the temperature of the auxiliary mined temperature.

heating element for regulating the temperature of the main heating element.

it. An electrical heating system comprising a main heating element having a negative temperature resistance coefficient, the temperature of which is to be regulated, a transformer having a plurality of sections in the primary winding, an electrical circuit supplying the same, an auxiliary heating element out of thermal communication with the main heating element and receiving current in proportion to that supplied to the main heating element, means actuated in response to increases of the temperature of the auxiliary heating element for increasing the effective number of turns of the primary winding connected to the electrical circuit.

15. An electrical heating system comprising a main heating element having a negative -temperature resistance coeflicient, the temperature of which is to be' regulated, a transformer having a plurality of taps on the primary winding, an electrical circuit supplying the same, an auxiliary heating element out of thermal communication with the main heating element and receiving cur rent in proportion to that supplied to the main heating element, and means actuated in response to increases of the temperature of the auxiliary heating element for increasing the effective number of turns of the primary winding connected to the electrical supply circuit and for finally disconnecting the supply circuit from said transformer winding, upon the occurrence of a predeter- 16. An electrical heating system comprising a main heating element having a negative-temperature-resistance c eficient, the

, gamete temperatureof which is to he regulated, a transformer .having a plurality of sections in the primary winding, an electrical circuit supplying said transformer, an auxiliary heating element having a plurality of sections of different heating capacities out of thermal communication with the main heating element and receiving current in proportion to that supplied to the main heating element, electromagnetic switches for energizing portions of the primary winding of said transformer, and a plurality of doublecontact thermostats controlled by said auxiliary heating element, for actuating said electromagnetic switches in predetermined order to maintain a practically constant current in the main heating element.

17. An electrical heating system comprising a main heating element having a negative temperature resistance coeflicient, the temperature of which is to be regulated, a

transformer having a plurality of taps on,\ "the primary, winding, an electrical circuit supplying the same, an auxiliary heating element out of thermal communication with the main heating element and receiving current in proportion to that supplied to the main heating element, and means actuated in: response to increases of the temperature of'the auxiliary heating element for increasing the efiective number of turns of the primary winding connected to the electrical supply circuit and for finally disconnecting the supply circuit from said transformer winding upon the occurrence of a predetermined temperature-and for reconnecting said supply, circuit to said transformer when the temperature drops to a value lower than said predetermined value.

18. An electrical heating system, comprising a main heating element having a negaproportion to that supplied to the main heating element, electromagnetic switches for energizing sections of said transformer, circuit-closing relays for controlling said electromagnetic switches, and a plurality of thermostats controlled by said auxiliary heating element for actuating said circuitclosing relays to cause said electromagnetic switches to close and to open in predetermine d order to maintain a practically constant current in said main." heating element.

In testimony whereof, I have hereunto subscribed my name this 31st day of Jam,

. ORA A, co e-r, 

